The family Orthomyxoviridae comprises influenza A, B, and C viruses, and Thogoto- and isavirus (Cox et al., 2005; Lamb et al., 2001). Influenza pandemics in humans are caused by influenza A viruses. Influenza A viruses contain 8 single-stranded, negative-sense viral RNAs (vRNAs) that encode 10-11 proteins (Cox et al., 2005; Lamb et al., 2001). The viral replication complex comprises three polymerase proteins (PB2, PB1, and PA), encoded by the three largest genome segments, and the nucleoprotein (NP), which is encoded by segment 5. The M1 viral matrix protein and the M2 ion channel protein are both encoded by segment 7 and translated from unspliced or spliced mRNAs, respectively. Two proteins are also encoded by segment 8: NS1, derived from an unspliced RNA, counteracts the cellular interferon response and NEP, derived from a spliced RNA, is a nuclear export protein. NEP mediates the transport to the cytoplasm of newly synthesized viral ribonucleoprotein complexes (vRNPs, composed of viral RNA and the polymerase and NP proteins).
Influenza A viruses contain two surface glycoproteins, hemagglutinin (HA) and neuraminidase (NA), that are encoded by segments 4 and 6, respectively (Cox et al., 2005; Lamb et al., 2001; Wiley et al., 1997). The HA protein is the major viral antigen and mediates virus binding to sialic acid-containing receptors on the cell surface. It is synthesized as a single polypeptide and subsequently cleaved into HA1 and HA2 subunits. HA cleavage is required for infectivity (Garten et al., 1999), because it generates the hydrophobic N-terminus of HA2, which mediates fusion between the viral envelope and the cell membrane. The NA protein cleaves α-ketosidic linkages between a terminal sialic acid and the adjacent sugar residue. The removal of sialic acids from cell surface sialyloligosaccharides, as well as from the HA and NA proteins, facilitates virus release from the cell surface and prevents virus self-aggregation. For optimal virus replication, the receptor-binding activity of HA and the receptor-destroying activity of NA are balanced (Wagner et al., 2002).
Localized, annual outbreaks of influenza, also called ‘seasonal influenza’, are caused by ‘antigenic drift’, where point mutations in the HA (or HA and NA) proteins allow virus variants to evade the human immune response. During epidemics, infection rates of 10%-20% are typical but can reach 50% in small groups. An estimated 20,000-40,000 people die from influenza-related illness annually in the U.S., creating a substantial economic burden, estimated at $17.5 billion in 1998 in direct costs (i.e., medications, hospitalizations, and physicians visits) and an additional $5.4 billion in indirect costs (i.e., loss of productivity).
Influenza pandemics are caused by ‘antigenic shift’, that is, the introduction of new HA (or new HA and NA) subtypes into the human population. The lack of prior exposure to the new HA (or new HA and NA) subtypes creates a population that is immunologically naïve to the antigenic shift variants, resulting in extremely high infection rates and rapid, worldwide virus spread. During the last century, three influenza pandemics have occurred. In 1957 and 1968, the ‘Asian influenza’ and ‘Hong Kong influenza’ killed an estimated 70,000 and 33,800 people in the U.S., respectively. Both pandemic viruses arose from reassortment of human and avian strains. In 1957, H2 HA and N2 NA genes of avian origin were introduced into a human virus. In 1968, the H2 HA gene was replaced with an avian H3 HA gene. The 1918/1919 ‘Spanish influenza’ is the most devastating infectious disease on record. An estimated 20-50 million people died worldwide and life expectancy in the U.S. was reduced by 10 years. The causative agent of ‘Spanish influenza’ was an H1N1 influenza A virus, which may have been introduced into human populations from an avian species. Estimates for future pandemics range from about 300,000 to 10 million hospitalizations in the U.S. and from about 90,000 excess deaths in the U.S. to up to 360 million excess deaths worldwide. The human toll and economic burden of future influenza pandemics would thus be extraordinary.
Although highly pathogenic H5N1 viruses have not (yet) caused a human pandemic, their continued transmission to humans and high mortality rate in humans have made the development of vaccines to these viruses a priority. The first transmission of highly pathogenic H5N1 avian influenza viruses to humans occurred in Hong Kong in 1997, when 6 of the 18 individuals infected succumbed to the infection. Since 2003, highly pathogenic H5N1 avian influenza viruses have become prevalent in Southeast Asia and endemic in poultry in some countries in this region. By the spring of 2006, these viruses had also spread to Europe and Africa.
Currently, two general types of influenza vaccines are available: inactivated and live vaccines. Inactivated vaccines are safe, but they induce humoral, not cellular, immune responses. Thus, the efficacy of inactivated vaccines is limited. On the other hand, live vaccines are more efficacious than inactivated vaccines since they induce both humoral and cellular immune responses. However, the safety of live vaccines varies.
Thus, there is a need for improved influenza vaccines.